The present invention relates to a system for collecting data about a moving body with use of ultrasonic waves to produce signals representing the characteristics of the moving body, for example, to a system for collecting data relating to a traveling vehicle to detect the vehicle, prepare data as to the contour of the vehicle (measure the height of the vehicle) and distinguish the type of the vehicle based on the contour data.
Conventional systems for distinguishing various types of vehicles with use of ultrasonic waves include an ultrasonic wave transmitter disposed at a predetermined level above the road surface. Ultrasonic waves in the form of a pulse are intermittently transmitted from the transmitter with a specified period against the road surface, and the return waves are received upon reflection from the road surface or a vehicle. The time taken for the reciprocation of the ultrasonic waves is measured to detect presence or absence of the vehicle, measure the height of the vehicle or distinguish the type of vehicle based on the data as to the vehicle height. The ultrasonic waves are transmitted intermittently to avoid mixing of the transmitted waves and the reflected return waves. Thus, after the transmitted waves have been received upon reflection, the next waves must be transmitted. The period with which the waves are transmitted is therefore relatively long, which entails the problem that it is impossible to expect accurate detection of vehicles traveling at a high speed. For example, suppose the ultrasonic wave transmitter is installed at a level of 5 m from the road surface, and the sound velocity is 340 m/s. The time interval between the transmission and reception of the ultrasonic wave is then about 30 ms. Assuming that the velocity of the vehicle is 100 km/h and that the length of the vehicle is 4 to 5 m, it takes about 150 ms for the vehicle to pass the point of detection. Accordingly, even if the period of transmission of ultrasonic waves is 30 ms, the sampling points are limited only to a maximum of five for the vehicle traveling at 100 km/h. Since all the sampling data is not always effective, about three items of data will be actually effective. The height of the vehicle can then be measured only locally. With the vehicle data available thus limited to the items relating to the front or rear portion or the roof of the vehicle, there arises the problem that it is impossible to distinguish the type of vehicle accurately.